J, Johnstone 
385 
bothridial region. Their diameter varies very greatly so that some¬ 
times they are large and quite circular in transverse section, and possess 
very thin walls with very few nuclei, but at other times they are greatly 
contracted and appear as mere chinks in the tissues of the scolex, their 
walls then being thick and rugose in transverse section. Occasionally 
the canals may be so much contracted as to be almost invisible so that 
it is very difficult to trace them from section to section of a series. The 
varying diameter appears to be due mainly to the manner of fixation 
of the worm. The distribution of the main excretory canals in the 
bothridia is very complex since they form a close plexus here. The 
main canals branch repeatedly and the finer vessels are connected by 
numerous anastomoses. These smaller branches are most abundant in 
the neighbourhood of the central nervous system and in the marginal 
parts of the bothridia particularly underneath the suctorial faces of the 
latter. 
The Central Nervous System. There is greater difficulty in the 
investigation of the nervous system than in that of any other system of 
organs in the whole Cestode. This difficulty is, of course, one which is 
experienced by all students of the nervous system of invertebrata, and 
depends largely on the failure to obtain a thoroughly satisfactory 
staining reaction in the case of the fibres. But added to this we have 
in the case of the Cestodes an arrangement of nerve tracts and centres 
which is unfamiliar since it differs notably from the conditions in most 
other invertebrates ; and which appears to be remarkably complex when 
one considers that there is not the same necessity for coordinated 
movement on the part of the animal, while the environment is a very 
uniform one. Added to these difiSculties there is that of the lack of 
definiteness in the centres and nervous tracts. In most invertebrates 
one finds rigidly circumscribed, or encapsulated centres or ganglia, and 
compact nerve cords; but in Tetrarhynchus all the organs are embedded 
in the ubiquitous parenchyma, and sometimes the limit between this 
tissue and the organs which it surrounds is not a very well-defined one. 
General stains do not differentiate the nervous structures of this 
tapeworm in a very satisfactory manner; and in the only preparation 
which I have been able to make, involving tlie use of vom Rath’s 
fixative followed by staining in iron haematoxylin, the reaction is not 
much more useful. Most of the figures which I give here are based on 
one complete series of sections of a scolex taken from a worm which was 
killed in fresh water and fixed in dilute formalin. These sections were 
stained with methyl-blue-eosin, but probably because of the imperfect 
